qemu: Save domain status ASAP after creating qemu process

qemu: Save domain status ASAP after creating qemu process

When creating new qemu process we saved domain status XML only after the
process was fully setup and running. In case libvirtd was killed before
the whole process finished, once libvirtd started again it didn't know
anything about the new process and we end up with an orphaned qemu
process. Let's save the domain status XML as soon as we know the PID so
that libvirtd can kill the process on restart.